12.11: Exceptions and ensure, ifCurtailed Interaction
- Page ID
- 45969
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Now that we saw how exceptions work, we present the interplay between exceptions and the ensure: and ifCurtailed: semantics. Exception handlers are executed then ensure: or ifCurtailed: blocks are executed. ensure: argument is always executed while ifCurtailed: argument is only executed when its receiver execution led to an unwound stack.
The following example shows such behavior. It prints: should show first error followed by then should show curtailed and returns 4.
[[ 1/0 ]
ifCurtailed: [ Transcript show: 'then should show curtailed'; cr. 6 ]]
on: Error do: [ :e |
Transcript show: 'should show first error'; cr.
e return: 4 ].
First the [1/0] raises a division by zero error. This error is handled by the exception handler. It prints the first message. Then it returns the value 4 and since the receiver raised an error, the argument of the ifCurtailed: message is evaluated: it prints the second message. Note that ifCurtailed: does not change the return value expressed by the error handler or the ifCurtailed: argument.
The following expression shows that when the stack is not unwound the expression value is simply returned and none of the handlers are executed. 1 is returned.
[[ 1 ]
ifCurtailed: [ Transcript show: 'curtailed'; cr. 6 "does not display it" ]]
on: Error do: [ :e |
Transcript show: 'error'; cr. "does not display it"
e return: 4 ].
ifCurtailed: is a watchdog that reacts to abnormal stack behavior. For example, if we add a return statement in the receiver of the previous expression, the argument of the ifCurtailed: message is raised. Indeed the return statement is invalid since it is not defined in a method.
[[ ^ 1 ]
ifCurtailed: [ Transcript show: 'only shows curtailed'; cr. ]]
on: Error do: [ :e |
Transcript show: 'error 2'; cr. "does not display it"
e return: 4 ].
The following example shows that ensure: is executed systematically, even when no error is raised. Here the message should show ensure is displayed and 1 is returned as a value.
[[ 1 ]
ensure: [ Transcript show: 'should show ensure'; cr. 6 ]]
on: Error do: [ :e |
Transcript show: 'error'; cr. "does not display it"
e return: 4 ].
The following expression shows that when an error occurs the handler associated with the error is executed before the ensure: argument. Here the expression prints should show error first, then then should show ensure and it returns 4.
[[ 1/0 ]
ensure: [ Transcript show: 'then should show ensure'; cr. 6 ]]
on: Error do: [ :e |
Transcript show: 'should show error first'; cr.
e return: 4 ].
Finally the last expression shows that errors are executed one by one from the closest to the farthest from the error, then the ensure: argument. Here error1, then error2, and then then should show ensure are displayed.
[[[ 1/0 ] ensure: [ Transcript show: 'then should show ensure'; cr. 6 ]]
on: Error do: [ :e|
Transcript show: 'error 1'; cr.
e pass ]] on: Error do: [ :e |
Transcript show: 'error 2'; cr. e return: 4 ].